1. Field of the Invention
The present invention relates to a method of producing a photovoltaic element, more particularly a thin-film solar cell, and also to the construction of a photovoltaic element having good characteristics and producible in a high yield, more particularly the construction of a solar cell.
2. Related Background Art
In recent years, the study and development of various techniques for commercialization of solar power generation using solar cells have been pursued. For establishment of a system using solar cells and capable satisfying demands for power, it is necessary that the solar cells have a sufficiently high photoelectric conversion efficiency and improved reliability and are capable of being mass-produced.
Amorphous silicon solar cells attract attention because they are producible at a lower cost and have high mass-producibility in comparison with solar cells made by using crystalline Si or the like. This is for the reason that a semiconductor film or the like can be formed as a deposited film on a comparatively inexpensive substrate in the form of a belt such as a metal sheet or a resin sheet by using a source gas such as silane gas easily obtainable and decomposing the gas by glow discharge.
On the other hand, the study and development of microcrystalline silicon solar cells having a limited lowering in photoelectric conversion efficiency (the so-called photodegradation) at the time of irradiation with light in comparison with amorphous silicon solar cells and capable of being formed at a considerably lower temperature in comparison with crystalline silicon solar cells such as polycrystalline silicon solar cells are being promoted.
In a case where solar cells are used for supply of electric power for ordinary home use, an output of about 3 kW is required. If the solar cells have a photoelectric conversion efficiency of 10%, the total area of the solar cells will be 30 m2. In such a case, large-area solar cells are required. However, it is extremely difficult to make a solar cell without a defect through a large area for reasons relating to the producing process.
For example, in microcrystalline silicon having small grains randomly grown in a columnar shape, a low-resistance portion can be formed easily at the grain boundary and a shunt path can be formed easily through the grain boundary. Further, it is known that in a thin-film solar cell such as an amorphous silicon solar cell a pinhole or a defect may occur at the time of film formation of the semiconductor layer due to the influence of a dust or the like and act as a cause of a shunt which considerably lowers the photoelectric conversion efficiency and yield.
The cause of occurrence of a pinhole or a defect will be described in detail. For example, in the case of an amorphous silicon solar cell deposited on a stainless steel substrate, the substrate surface cannot be said to be a completely smooth surface; scratches or struck impressions exist in the surface, and a back surface reflecting layer having an unevenness structure is provided on the substrate for the purpose of effectively utilizing incident light. Therefore, it is difficult to completely cover this surface with a thin-film semiconductor layer such as an n-or p-layer having a thickness of about several ten nm.
In a case where a portion of a semiconductor layer existing between a first electrode (lower electrode) and a second electrode (upper electrode) is lost due to a pinhole or the like and the lower electrode and the upper electrode are in direct contact with each other, or in a case where the semiconductor layer is not completely lost at a portion but a low-resistance shunt exists at that portion, a current generated by light flows in the upper electrode parallel to the surface thereof and flows into the low-resistance shunt portion, thereby causing a current loss. If such a current loss occurs, the open circuit voltage of the solar cell is lowered. In amorphous silicon solar cells, the sheet resistance of the semiconductor layer itself is ordinarily high and, therefore, a transparent upper electrode covering the entire semiconductor surface is required. Ordinarily, a transparent electrode layer such as SnO2, In2O3 or ITO (In2O3+SnO2) film excellent in transparency to visible light and in electrical conductivity is provided. Therefore, the current flowing into a small defect will considerably be large. Further, in a case where the position of a defect is remote from a grid electrode provided on the transparent conductor layer, the resistance to the current flowing into the defective portion is large and, therefore, the current loss is comparatively small. On the contrary, in a case where a defective portion exists below the grid electrode, the current loss due to the defect will be larger.
On the other hand, at a pinhole-like defective portion, not only leakage through the defective portion of electric charge generated in the semiconductor layer occurs but also a phenomenon occurs in which an ionic substance is generated by the interaction with water if any, the electrical resistance of the defective portion will thereby be lower gradually with the passage of solar cell operating time, and degradation of the characteristics including the photoelectric conversion efficiency will result.
In a case where the above-described shunt occurs, the current loss can be reduced by directly removing the defective portion or the pinhole or removing the upper electrode about the shunt portion.
As a method of directly removing a defective portion, a method of burning off a defective portion of a solar cell by using a sufficiently high reverse bias not higher than the breakdown voltage is known (see, for example, U.S. Pat. No. 4,166,918).
As a method of selectively removing the upper electrode about a shunt portion, a method of removing the upper electrode about a shunt portion of a solar cell by etching in such a manner that the solar cell is immersed in an acid, salt or alkali electrolyte and a bias is applied between the solar cell and a counter electrode is known (see, for example, U.S. Pat. No. 4,451,970; Japanese Patent No. 2921802; Japanese Patent Application Laid-Open No. H11-233802; and Japanese Patent Application Laid-Open No. 2000-49370). Studies have already been made of conditions including the kind, concentration and specific conductivity of the electrolyte, the bias voltage range, the current density range in which the bias flows through the electrolyte or the solar cell when applied, the bias application time according to the film thickness of the transparent electrode layer, and application of a pulse voltage in a stepwise manner (in rectangular form).
However, the above-described method of burning off a defective portion has a problem that since a high reverse bias is applied to the solar cell, there is a possibility of damage to the normal portion other than the defective portion when the defective portion is burnt off, and it is difficult to control the process.
The method of applying the desired pulse voltage in a stepwise manner (in rectangular form) in an electrolyte is advantageously effective in selectively removing the upper electrode about a shunt portion but has a problem that when the applied voltage is abruptly reduced to a voltage at which etching reaction is not caused, e.g., 0 V, a large negative current (C·dV/dt where C is the electrical capacity between the solar cell and the counter electrode and dV/dt is a time differential of the applied voltage, hereinafter referred to as “voltage gradient”) flows into the solar cell to break a weak portion of the solar cell other than the shunt portion, thereby increasing the shunt path.